Prior to flight testing an aircraft structure or any part of a flexible structure, flutter wind-tunnel tests are required to search for AE/ASE instabilities of the structure's configurations. Flutter wind-tunnel testing is an expensive and complicated process because it involves the design and fabrication of a scaled-down aeroelastic structural model, which is a scaled-down version of a real structure, and also because wind-tunnel time is costly. Flutter wind-tunnel tests are necessary because if a flutter instability is not found prior to the flight test then major instabilities may occur during the flight tests which can make the structure extremely unstable, hence endangering the aircraft structure and passengers. Unfortunately, creating a scaled-down version of a real structure may introduce discrepancies in the structural characteristics of the scaled-down structure when compared to the real structure. For example, the control surface actuators with corresponding stiffness and damping, are difficult to represent accurately in the scaled-down structure. Also, the incorporation of the accurate modal damping in the scaled-down structural model is almost impossible. These discrepancies between the actual structure and the scaled-down versions can lead to uncertainties in the measured aeroelastic instability boundary. In addition, during flutter wind-tunnel tests the wind-tunnel walls interfere with the test results.
It is very costly to include structural nonlinearities such as friction, free-play, etc. in the scaled-down structural model. And even when these structural nonlinearities are included in the scaled-down version of the structure they will never be identical to the real structure. These structural nonlinearities can have a major impact on the aeroelastic characteristics of the aircraft. The measurement of the coupling between the flight control system and the aeroelastic system to search for ASE instabilities of the aircraft is another critical design requirement that can be accomplished by performing a flutter wind-tunnel test in the presence of flight control laws. However, performing this aeroservoelastic (ASE) measurement in the wind tunnel is also a very expensive process, thus the ASE analysis largely relies on flight tests.
If the flutter wind-tunnel test was able to use the real-structure then the control laws would automatically be included, since the real structure has control laws included in it to be able to fly. With the use of a real-structure there would be no scaling discrepancies between the tested structure and the actual structure. The impact of the structural nonlinearities on aeroelastic stability can automatically be included. If the flutter test could be conducted without a wind-tunnel then the unsteady aerodynamics generated computationally would be interference-free from the wind-tunnel walls.
In order to be able to conduct a flutter test without a wind-tunnel, a ROM (Reduced Order Model) of the unsteady aerodynamic model is needed. This unsteady aerodynamic ROM represents an aerodynamic transfer function that inputs the physical structural deformation and outputs the aerodynamic forces. Thus, this requirement immediately rules out the Computational Fluid Dynamics (CFD)-based ROM's because all CFD-based ROM's involve some type of modal approach that assumes the structural mode shapes are known.